An in situ telecentric height monitoring system was employed as a standard characterization tool for in situ height monitoring of nanotube forests. We demonstrated that the system possesses a wide dynamic range beyond the centimeter scale, high resolution of 1 mu m, free from routine maintenance, simple installation, and use in contrast to previous methods. These features were highlighted by the monitoring of a 5 mm tall forest, the examination of the effect of gas species on growth rates, and the automatic control forest height. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.2987480]